Orchids, in fact all plants, require nutrition just like us to grow and reproduce. Plants take up nutrition in form of inorganic compounds present in the soil. In other words, the soil is composed of different types of mineral salts; like carbonates of calcium or sodium, different chlorides etc. Out of these soil minerals. Plants absorb some of minerals in large quantities and these are called macro-nutrients. There are others which are required and absorbed in very small amounts, called the micronutrients. Both types of nutrients are important for the plant and their deficiency in the plants (orchids) can be judged by deficiency symptoms. Such deficiencies can lead to poor or deformed growth, pest and diseases attacks.
Nutritional requirement for each type of orchid is different. and is determined by the various factors such as morphology, growth habit and consequent needs, prevailing environmental conditions.
When trying to apply fertilizers to orchids, first be aware of the particular needs of that orchid. If fertilizer instruction were supplied along with the orchid, read and follow those instruction carefully.
Most commonly used fertilizers are those which contain N (Nitrogen), P (Phosphorous), and K (Potassium) in different proportions. Fertilizers such as urea lead to an increase in leaf number. There are reports that fertilization does not have much effect on the flowering time, floral size or number of flowers.
A fertilizer having a higher proportion of Nitrogen as compared to Phosphorus, has a better effect on overall growth of the orchid.
There are two types of fertilizers: organic fertilizers and mineral or inorganic fertilizers. Sticking to just one type of fertilizer can harm the orchid more than doing it good. It is better to use a combination of organic and mineral fertilizers. This will help in supplying both kinds of nutrients (macro as well as micro) to the orchid. You can use a 1:1 mixture of organic and mineral fertilizers for your orchids.
Organic fertilizer is best when made at home. It is quite easy to prepare an organic fertilizer of leaf mold. Collect the fallen leaves in a pit and keep turning the leaves after an interval of few days. Turning leaves into leaf mold is work of bacteria and tiny insects. After about six months, when the texture and colour of the leaves changes to a dark clumpy stuff, your fertilizer is ready to be used in pots.